Conventionally, a magnetic resonance measuring apparatus is used to detect a substance that contains unpaired electrons. This measuring apparatus irradiates an object to be measured in a resonator with microwaves and measures resulting electrical spin resonance. Since the object is accommodated in the resonator, the measuring apparatus is subject to a number of restrictions. For example, the magnetic resonance measuring apparatus has the following problems. First, evaluable sizes depend on the size of the resonator. Secondly, the object to be measured can be evaluated at a single frequency only. Thirdly, the reception sensitivity is reduced depending on the bandwidth and a loss in coupling to the resonator.
Since a cavity resonator that resonates with a half-wavelength is used in a conventional magnetic resonance measuring apparatus, the half-wavelength is 15 cm or less even if the measurement frequency is in an L-band range (1 to 2 GHz). Consequently, only a small object to be measured can be accommodated in the resonator.
Conventionally, moreover, the loss in coupling to the resonator can be reduced by enhancing the coupling. Since the resonance characteristics become broad in this case, however, a large amount of noise enters a receiver system. If the coupling is reduced, in contrast, narrow-band characteristics can be obtained, so that the coupling loss increases although the noise that enters the receiver can be reduced. Thus, the reception sensitivity of the system is reduced.